Help! Fish dying and I don’t know why!

[duanes]

How many of, and what species did the tank start with?
are any not dying?
What is left?
What are other water parameters?
pH, ? hardness ? Alkalinity?
And what are the parameters before and after water changes?
And the question posed about surface agitation is relevant, is your flow content breaking surface tension?
The Geos you mention are all species that prefer a moderate to strong laminar flow, and high dissolved oxygen content.

 

[Fishman Dave]

Where does your water change water come from?
I would also agree that even for ro water mixed with warm water a nitrate level of 0 would surprise me. Let alone a mature tank after a water change.
watch out for scented candles or similar lit in not only the room but adjacent rooms. This is more common than you would think in fish deaths.

 

[phreeflow]

Sorry to hear your troubles. Erratic swimming, dark colour, fast deaths usually indicate chemical toxicity.

Fly sprays? Pest treatments? Unwashed hands in tank? Change in supply water?

If your unsure what's happening the golden rule is a large water change which you have alreaady done.

That is a possibility also. Knew a guy who lost his entire tank of plecos. His only filter was a sponge filter and although he covered the tank and taped all openings, he lost them all when they tented his home and sprayed for termites. His logic was that the air pump sucked in the pesticides and bubbled them through his tank. Any chance you used chemicals around the tank? If so, you may want to do massive water changes and add carbon